1. Field of the Invention
The present invention relates to a variable compression ratio control device for an internal combustion engine, including a mechanism for varying the compression ratio of an engine in response to engine driving conditions.
2. Description of the Related Art
In an internal combustion engine, desirably the compression ratio is increased to bring an improved fuel efficiency and an increased axial torque. Nevertheless, any increase of the compression ratio is limited because it will increase the probability of knocking in the engine when the gas in the combustion chamber is adiabatically compressed and the temperature of the gas is high. Knocking occurs under a high engine load more than under a low engine load, and therefore, desirably the compression ratio is varied in accordance with the engine load, i.e., the compression ratio is high under a low engine load and low under a high engine load.
To satisfy these requirements, various compression ratio varying devices have been proposed for an internal combustion engine in, for example, Japanese Unexamined Patent Publication No. 62-142860 and Japanese Unexamined Patent publication No. 63-105244, and in the above compression ratio varying devices, an ignition timing is determined from the engine load and from an actual compression ratio detected by a compression ratio detecting means such as a combustion pressure sensor or a piston position sensor for detecting the TDC of a piston, of wherein the level of the output signal of the sensors corresponds to the compression ratio.
In detail, in a device which includes a mechanism for varying the compression ratio in two stages, e.g., high and low, if the engine is driven at a high compression ratio, the ignition timing is determined from an ignition map for the high compression ratio, in which the ignition timing is usually delayed, to thereby decrease the tendency toward knocking. On the other hand, if the engine is driven at a low compression ratio, the ignition timing is determined from another ignition map for the low compression ratio, in which the ignition timing is usually advanced, to thereby improve the combustion.
Nevertheless, in the conventional control devices described above, if the compression ratio detecting means such as a combustion pressure sensor or piston position sensor have a failure by malfunction and thus do not operate normally, i.e., if the above sensors generate an output signal level corresponding to the low compression ratio although a high compression ratio state actually exists, the control device will erroneously employ the ignition map for the low compression ratio in which ignition timing is usually advanced, and thus the drivability will be adversely affected by a loss of power from the engine and it is possible for the engine to suffer serious damage due to the resultant increase in the knocking.